Before 1861, the Italian peninsula was a patchwork of independent kingdoms, duchies, and papal states. The Risorgimento ('resurgence') movement changed everything when Giuseppe Garibaldi led his famous 'Thousand' volunteers to unite the south with the north. Working with the diplomatic Count Cavour and King Victor Emmanuel II of Sardinia-Piedmont, they transformed these separate states into the modern nation of Italy. While most of unification was complete in 1861, Rome would join the new kingdom in 1870.
Impact on Citizenship
This event marks the beginning of modern Italian citizenship. Only people with ancestors born after this date can claim Italian citizenship, as this was when Italy officially became one country. If your family was already living in Italy when unification happened, they automatically became Italian citizens, but having family from Italy before unification doesn't qualify you for citizenship today.